Modulation



Dec. 24, 1940.

L.. W. HUSSEY NVENTOP.

By .WHUSSE'Y i Patented Dec. 24,1940

UNITED STATES MoDULA'rIoN .Luther W. Hussey, Madison, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application september 21, 1939, 'serial No. 296,729

11 Claims.

'This invention relates to carrier wave signaling systems and more particularly to modulating arrangements for such systems.

in object of the invention is to improve the economy and simplicity of modulating arrangements known heretofore and to neutralize or reduce the deleterious effects of distortion products that tend to appear. e

It is well known that if an electric wave of a given frequency f and another wave of frequency s are applied to an element having non-linear current-voltage characteristic there are generated waves of other frequencies, which generalg5 ly comprise fis, 2f, 2s, 2fi-s, fi2s, 3f, 3s, 3fis,

etc., the relative amplitudes of the waves generated beingv dependent on the particular nonlinear characteristic of thefmodulating element. In a carrier signaling system, where f would so represent the carrier wavev and s the signal, it is desired to segregate one ofthe modulation products, as they are called, of the form nfis, where n is an integer, usually unity, so that that product can be transmitted to the receiving station 25 and the other modulation products suppressed.A

Suppression of these other products is especially necessary in a multiplex carrier signaling system for they may fallin the frequency range oc- 80 cupied by other carrier signals, thus giving rise to lnterchannel interference or cross-talk.

In accordance with the present invention. un-

wanted Amodulation products in the outputof a modulator are reducedby generating like modulation products in a separate modulator and com-1 wanted products of the first modulator in such manner that the desired signal sidebands are not affected. Ihe 'separate modulator maybe used osolely for-the generatiorrofthe neutralizing products or, in accordance with a modincation, it may rier signaling channel.

'I'he nature of 'the present invention and. its various features, objects and advantages will be more fully apparent from a consideration of the following description of the preferred embodiments illustrated in the accompanying drawing: Fig. 1 shows a modulation arrangement fora o multiplex carrier signaling system utilizing a, separate modulator in accordance with the invention for neutralizing certain disturbing modulation products; and

Fig. 2 shows schematically a modification in be used simultaneously Vprovide another cari5 which one modulator serves both to provide a sig- 1 binlng'them in opposing relation with the un? naling channel and to neutralize interchannel modulation.

Although any modulator tends to generate the various .modulation products noted hereinbefora there are some modulators which operate lin- 5 herently to suppress certain of these modulation products. Those products not suppressed in- Y herently may be and usually are eliminated by passing the wave output of the modulator through a lter'that passes the desired signal sidebands 10 but which attenuates the undesired products to a negligible value.. A modulator of `this type has been chosen for the embodiments of the present invention illustrated in the drawing inasmuch as it not only will serve for purposes of exposition of the basic principles ofthe invention but it also makes possible by application of the invention a multichannel modulating arrangement requiring' no filters.

Referring now to Fig. 1, there is represented a modulating arrangement for a three-channel carrier system in which signals s1, s2 and s3 aret'aken Internally of each bridge the asymmetric elee 1 ments' are connected in a closed loop and are like poled with respect to their loop connection. Each input transformer is connected across a pair of diametrlcally opposite terminals of its associated 40 :bridge and the other pair of terminals of the bridge is connected to the associated output transformer. The carrier-wave sourcel 4, 5, Sisconnected between the mid-points o: the secondaryand primary windings of the transformers l and 9, respectively.

It has been shown that in a modulator of the kind described, when the amplitude of the carrier wave is made much greater than that of the signal wave, all odd order modulation products are inherently suppressed by virtue of the balanced arrangement of the circuit, that the principal modulation product i-n the'output circuit is the second order double side-bands with frequencies are in the harmonic relation f, 2f, 3f. These harmonically related carrier frequencies may be.

for specic example 8, 16 and 24 kllocycles per second. In the case assumed the distortion component 3f1; is1 falls squarely in the frequencyA range occupied by the desired signal side-bands farsa. It may readily be shown also thatall other principal modulation products of the form 52nd-l) fi-.s liall,. outside of the frequency range occupiedby thethree carrier channels. The application ofthe-:present invention to the typical modulating arrangement disclosed will accordingly be directed and coniined to the reduction or neutralization of the distortion productfiisi.

InVV accordance with the invention as illustrated in Fig. 1,*the modulatorto winch signal s1 is ap-. plied is parallelled by anotherv or auxiliaryy modulator comprising a modulator bridge M1. and an output transformer 9' that have substantially the same electrical characteristics as the corresponding circuit elements M1 and 9. Duplication of the input transformer 8 is unnecessary'and. the latter. may :serve for both modulators. Carrier waves of frequency f4=3f1 are applied to the second modulator from a source 5' connected between the secondary mid-point of transformer 8 and the primary midepoint of.

transformer .9'. I'he two carrier Wave sources 4 and '6' areV electrically or mechanically geared together in exact harmonic ,relation and so phased that carrier wave f4 passes through zero value whenever' the instantaneous value of carrier wave f1 is zero. In practice all four carrier waves lcould well be derived Yfrom a harmonic gen- `erator comprising a diode or other reactance- 4d free non-linear resistance device so that the desired phase relation between carrier Waves fvand f4 would' inherently obtain( If the phase characteristics of the parallelled modulators are not exactly alike, itis permissible and desirable to )50- compensate for the difference by departing from input terminals of modulator bridge M1' and in- B5' put transformer 8,.and on adjusting the'attenu- Aation of the pad to an optimum value equalizing the amplitudes of the opposing products their neutralization is effected. Although the equalizer function could be accomplished by various 70 alternative means, that illustrated is preferred inasmuch as-by reducing theamplitude of the waves applied to the auxiliary modulator the varions unwanted modulationproducts it tends to produce are reduced in greater proportion than' 75the wanted product flier.

The modulatingarrangement described with reference to Fig. l may be modified in the manner indicated in Fig. 2 so that one of the three channel modulators performs the additional function of generating the modulation products desired for effecting neutralization of interchannel cross-talk. The modification in part consists an attenuator pad 'I2 connecting signal circuits I and 3 sothat an attenuated fraction of signal s1 is applied together with signal s3 to the modulator MOM associated with the latter. A hybrid coil I3 is interposed in the connection described so that circuits vBand I I are in conjugate relation with each other but in energy transfer relation with the modulator MODs. The carrier Wave sources B and 8 are coupled together in any suitable manner to maintain the same phase relation as was prescribed for sources 4 and 6 in Fig. 11 It will be readily perceived that the output of modulator MOD; includes not only the principal desired signal side-band farsa but also a second order modulation product farsi falling in the same frequency range. In accordance with the procedure outlinedvhereinbefore, viz., thatof proper poling of the output terminals of MOD: and adjustment of the attenuator pad I2 as described with reference to Fig. 1, the last-men- -tioned product fsi-si will be combined in neutralizing relation with the rdistortion product lShi-s1 originating in the modulator4 MOD; as-

sociated with signal circuit I.

Although the present invention has been described with" reference to its application to the neutralization of a particular distortion product in a second order modulator of specic type, it .will be understood that the invention and the r principles underlying the invention have widely diversified application within the spirit and scope of the appended claims.

What is claimed is: l

1. In combination, two modulators that are substantially alike ln'their modulating characteristics; a common source of signals and a coinmon output circuit for said modulators, means in the elimination of the auxiliary modulator of Fig. 1 and the provision of a circuit iI- including for supplying respective carrier waves of dilerent frequencies and fixed relative phase to saidl modulators, said carrier frequencies being so related that an unwanted modulation product of a given order produced by one of said modulators is in phase opposition with a principal modulation product of a different order produced by the other of said-modulators, and means for equalizing the amplitudes of said modulation products, whereby they tend to neutralize each other.

2. In combination, two modulators, a common maintaining-said carrier waves in fixed harmonic phase relation with each other, one of said modulators producing a wanted doubleside-band and unwanted distortion products of order differing from that of said double side-band, said carrier waves being so' relatedin frequency that a prin- -cipal .modulation product produced by the other of said modulators coincides inA frequency -and composition with one of said distortion products,

means for equalizing the amplitudes of said modulation .and distortion products, and means for combining them in phase opposition in said comand rfatsm asados? comprising a iirst modulator and a second modu-` lator, means for applying to said nrst 4modulator for intermodulation therein a signal si and a carintermodulating a second signal-. v and a carrier wave of frequency fr such thatt principal rsultings modulationrproduct dat): coincides at least in part with the frequency range allier. 17. q and 1' being integers, means for`combining :said distortion product and the said principal modulation product of said second modulator in mutually opposing relation whereby they tend to neutralize each other, andra common load circuit for said principal modulation products plins1 4. A system in accordance with claim. 3 comprising means substantially attenuating `the said signals as applied to saidsecqnd modulator rela- Y tively to said signals as applied to said nrst mod- A ulator.

prising a third modulator for the said modulation of said second signal.

6: A system in accordance with claim 3 in which the said modulation of said'second signal is effected-in said second modulator.

7. In combination, a pair of double-balanced.

modulators having substantially the same elecv nals and a common output circuit for trical characteristics, a common source1 of sigsaid modulators, means for supplying respective carrier waves to said modulators. the frequencyot one of said carrier waves being three times that of' the ouer and :sustained in axes harmonic phase relation therewith,'whereby the second order signal side-bands from one-of saidv modulators coincides in frequency with the fourth order signal vside-bands fromA the other, means for .equaiizing 4the amplitudesfof said second and fourth order side-bands, and m'eans for combining them injmutually opposing relation Vin sai common output circuit. 4

8. A combination in accordance with claim 7 in which said equalizing means comprises means for adjusting the amplitude of the signal applied to one of saidmodulators. f

9. In a multichannel carrier signaling system,` two modulators yeach of a type such that modulation products ofthe form 21u23 are suppressed but in which modulation products of the form 31:;3 tend to appear, means for'supplying said modulators with respective carrier waves. the fre'- quency of the carrier waves supplied to one o said modulators being three times the frequency of the carrier waves supplied to the other, 4means for supplying the same signal 5to both of said modulators, a common output circuit for said modulators, means for adJusting the amplitude of the, 3111s products of said one modulator to 'equality/with the amplitude of the nis products o f said other modulator,r means for combining I said two lastfmentioned products in phaser oppot5. A system in accordance with claim 3 co sition in said output circ t. means for translatling a different signal tou e frequency range of f said last-mentioned products, and means vfor apc.

plying said diiferent signal so-translated to output circuit. 10. A combination in accordance with claim 9 in which said other modulator eifects the frequency translation of said diiferentf signal.

said

-11. a combination 1n' accordance wiui claim 9 I including a separate modulator for eifecting the frequency translation ofsaid di-iferentsiznal.

Lu'rrmaw. HUssEY. 

